Semiconductor memory devices may be classified into volatile semiconductor memory devices and nonvolatile semiconductor memory devices. Volatile semiconductor memory devices have a high read/write speed but have a disadvantage of losing their stored data when their power supplies are interrupted. Nonvolatile semiconductor memory device retain their stored data even when their power supplies are interrupted. Thus, nonvolatile memory devices are used to remember contents that have to be preserved regardless of whether power supplies are supplied or not.
Examples of nonvolatile semiconductor memory devices include a mask read-only memory, a programmable read only memory (PROM), an erasable programmable read only memory (EPROM), an electrically erasable programmable read only memory (EEPROM), etc.
A typical example of a nonvolatile memory device is a flash memory device. A flash memory device is being widely used as a voice and image data storage medium of information devices such as a computer, a cellular phone, a PDA, a digital camera, a camcorder, a voice recorder, a MP3 player, a personal portable terminal, a handheld PC, a game machine, a fax scanner, a printer (hereinafter it is referred to as ‘host’).
As a high integration requirement for a memory device increases, multi-bit memory devices that store multi bit in one memory cell are becoming more common.